如何在DLR中实现三元运算符 [英] How to Implement the Ternary Operator in the DLR

问题描述

我正在使用DLR在C#中实现语言解释器，并且在使用三元运算符时遇到了一些麻烦.至此，我已经实现了基本的函数声明/调用，如下所示:

I am implementing a language interpreter in C# using the DLR, and I'm having some troubles with the ternary operator. At this point, I have basic function declarations/calls implemented, like so:

F := (x) -> x + 1
F(1)   # returns 2

对于函数体是一系列表达式，我没有问题-总是返回最后一个表达式的值，而且我确保解释器中的所有情况至少返回 something 作为副作用.我现在正在尝试实现三元运算符(?:).我正在渲染的表达式树看起来像这样:

I've not had a problem with a function body being a sequence of expressions -- the value of the last expression is always returned, and I've made sure all cases in the interpreter return at least something as a side effect. I'm now trying to implement the ternary operator (? :). The Expression tree I'm rendering looks like this:

work = Expression.IfThenElse(                                   
    Expression.IsTrue(Expression.Convert(work, typeof(Boolean))), 
    trueExp, 
    falseExp);

其中trueExp和falseExp都是有效表达式.

where trueExp and falseExp are both valid expressions.

问题似乎是IfThenElse表达式未返回值，因此，基本上，即使trueExp和falseExp正在构建表达式树，IfThenElse表达式的最终结果始终为null.除了制作一个Runtime函数并显式调用它之外，还有没有办法使用DLR实现三元运算符? (即:是否执行IfThenElse并在true和false子句中返回实际值的表达式?)

The problem seems to be that the IfThenElse expression does not return a value, so basically even though trueExp and falseExp are building expression trees, the end result of the IfThenElse expression is always null. Short of making a Runtime function and explicitly calling it, is there a way to implement the ternary operator using the DLR? (ie: an Expression. that does the IfThenElse and returns the actual values in the true and false clauses?)

我希望解析的内容如下:

What I hope to parse is something like:

F := (x) -> (x = 1) ? 4 : 5
F(1)   #4
F(2)   #5

但是现在，由于上述问题，当编译到程序中时，此方法始终返回null.

But right now this always returns null when compiled into a program, because of the problem outlined above.

我将不胜感激，这非常令人烦恼！

I'd appreciate any help, this is quite vexing!

推荐答案

Expression.IfThenElse是if (...) ... else ...;构造，而不是三元运算符.

Expression.IfThenElse is an if (...) ... else ...; construct, not the ternary operator.

三元运算符是 Expression.Condition

The ternary operator is Expression.Condition

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